Using a background image to model in Blender
Press N to open the properties panel on the right side of the 3D view (if it is not already open)  Then scroll down till you see backround images and click add image.  To see the image you must be in orthographic mode and a side view, pressnumpad 5 to toggle orthographic/perspective mode and numpad 1/3/7 to go into a side view.